Add schema metadata validation

[mindsocket]

We currently embed metadata (hierarchy, aliases, allowSkipLevels, allowSelfRef, rules) in schema ._metadata. This metadata is not validated by JSON Schema and could benefit from explicit validation.

Proposed Options

1. Extend JSON Schema: Create a schema definition that extends the standard JSON Schema to explicitly validate metadata fields while maintaining compatibility with existing JSON Schema validators.

2. Separate metadata file: Create a parallel metadata file (e.g., schema-name.metadata.json) that would be validated independently.

Requirements

• Validate that hierarchy is a non-empty array of strings
• Validate that aliases is an object mapping string → string
• Validate that allowSkipLevels is a boolean (default false)
• Validate that allowSelfRef is an array of strings
• Validate that rules follows the RulesMetadata structure (validation/coherence/best-practice arrays)
• Maintain backward compatibility with schemas that don't define all metadata fields

Preference

Per discussion, preference is for Option 1 (extend JSON Schema) to keep metadata and schema in one file, but this requires careful design to avoid breaking JSON Schema validators.

[mindsocket]

This issue depends on #33 (Support full cross-schema resolution of refs) - to validate schema metadata, we need to be able to resolve all references to ensure metadata is complete and consistent.